Conventionally, as media access control in a wireless LAN system, the access control defined by an IEEE (The Institute of Electrical and Electronics Engineers) 802.11 method and the like have widely been known. A detailed description of the IEEE 802.11 method is provided in International Standard ISO/IEC 8802-11:1999(E) ANSI/IEEE Std 802.11, 1999 Edition, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PRY) Specifications and the like.
An access competition method according to a conventional IEEE802.11 method is explained using FIG. 12. In the IEEE802.11 method, four kinds of IFS (Inter Frame Space) are defined as a packet interval. Hereupon, three kinds thereof are explained. As the IFSs, SIFS (Short IFS),
PIFS (PCF IFS) and DIFS (DCF IFS) are defined in order of length from a shorter one to a longer one. In the IEEE802.11, CSMA (Carrier Sense Multiple Access) is employed as a basic medium access procedure, in which a back-off timer is operated over a random period of time while monitoring a state of medium before a transmitter transmits something and a transmission right is granted only when a transmission signal is not present during the period.
When a normal packet is transmitted in accordance with the procedure of CSMA (called DCF: Distributed Coordination Function), first the state of medium is monitored for a period of DIFS after transmission of some packet is completed, a random back-off is performed if a transmission signal is not present during this period, and the transmission right is to be granted if a transmission signal is not present further during the period. On the other hand, when a packet having an exceptionally high urgency such as ACK which is an acknowledgement response signal is transmitted, it is allowed to transmit after the SIFS packet interval. Accordingly, it is possible to transmit the packet having the high urgency earlier than a normal packet transmitted in accordance with the CSMA procedure.
This is the reason why different kinds of IFS are defined, and with respect to the conflict of packet transmission rights, the order of priority is determined depending on the IFS whether it is SIFS, PIFS or DIFS.
Next, IEEE802.11a which is an extended standard of the IEEE802.11 is referred to as an example and an explanation is made to a frame format (packet format) using FIGS. 13 and 14. FIG. 13 is a diagram showing the frame format according to the IEEE802.11a. A preamble is added to the head of each packet to indicate existence of the packet. According to the standard, a known symbol pattern is defined as the preamble, and a receiver judges based on the known pattern whether a received signal is entitled to be the preamble or not.
Subsequently to the preamble, a signal field is defined. Information required for decoding an information portion of the packet is stored in the signal field. The information required for decoding the packet is called a PLCP header (Physical Layer Convergence Protocol header), and the PLCP header includes a rate field indicating a transmission rate of an information portion (in addition, a service field which is a part of the PLCP header is also included, however hereinafter it is generally termed the information portion in order to simplify an explanation), a data length field indicating a length of the information portion, a parity bit, a tail bit of an encoder and the like. The receiver performs decoding operations of the subsequent information portions based on the result of decoding the rate and length fields of the PLCP header.
Encoding resistant to noise is applied to a signal portion which stores the PLCP header, and transmission is performed at a rate equivalent to 6 Mbps. On the other hand, the information portion in an ordinary packet is transmitted in a transmission rate mode with which the highest bit rate is provided within the range where error as little as possible is generated in accordance with SNR or the like in the receiver. As shown in FIG. 13, total eight kinds of transmission rate mode, which are 6, 9, 12, 18, 24, 36, 48, and 54 Mbps, are defined in the IEEE802.11a.
Therefore, in the case where a transmitting and receiving device is located nearby, a transmission rate mode having a high bit rate is selected, and it may not possible to decode the information in a communication station located in the distance. The information portion is transferred as PSDU (physical Layer Service Data Unit) to a link layer which is an upper layer.
FIG. 14 is a diagram showing a frame field of the PSDU. Although some frame types are defined in the IEEE802.11, hereupon only three kinds of frame, which are necessary for the explanation, are explained.
A frame control field and a duration field are defined in common in each frame. Information indicating a kind, use and the like of the frame is stored in the Frame Control field. Moreover, information on use for NAV (Network Allocation Vector), which is explained in detail later on, is stored in the Duration field, and a period of time until a transaction of the packet is completed is written therein. In a data frame, other than the above, there exist four address fields to specify a transmission source, a destination communication station and the like, a sequence field (SEQ), a frame body which is net information to be provided to an upper layer and FCS (Frame Check Sequence) which is a checksum. In an RTS frame, other than the above, there exist a receiver address (RA) which indicates a destination, a transmitter address (TA) which indicates a transmission source and FCS which is the checksum. In a CTS frame and an ACK frame, there exist, other than the above, RA which indicates the destination and FCS which is the checksum.
An RTS/CTS procedure in the IEEE802.11 is explained using FIGS. 11 and 15. In a wireless LAN network of an ad hoc environment, it is generally known that a problem of hidden terminals occurs and CSMA/CA according to the RTS/CTS procedure is known as a methodology for solving much of this problem. This methodology is employed also in the IEEE802.11.
An example of an operation in the RTS/CTS procedure is explained using FIG. 11. FIG. 11 shows an example in which some information (Data) is transmitted from a communication station STA0 to a communication station STA1. Prior to transmission of actual information, the communication station STA0 transmits an RTS (Request To Send) packet to the communication station STA1 which is a destination of the information in accordance with the CSMA procedure. When the communication station STA1 has received this packet, a CTS (Clear To Send) packet to feed back the fact that the RTS has been received is transmitted to the communication station STA0. When reception of the CTS is performed without failure in the communication station STA0 which is a transmitting side, it is considered that a medium is clear and an information (Data) packet is immediately transmitted. After the reception is completed without failure in the communication station STA1, an ACK is returned to complete a transmission and reception transaction for one packet.
Operations occurred in this procedure is explained using FIG. 15. In FIG. 15, an explanation is made to an example in which communication stations STA2, STA0, STA1 and STA3 exist and only communication stations adjacent to each other are located within the range of reaching an electric wave. Further, it is assumed to be the case in which the communication station STA0 transmits information addressed to the communication station STA1. After confirming that a medium is clear for a predetermined period of time (from time T0 to time T1) in accordance with the above described CSMA procedure, the communication station STA0 starts to transmit an RTS packet addressed to the communication station STA2 at the time T1. Information indicating that the packet is RTS is written in a frame control field of the RTS packet, a period of time until a transmission and reception transaction of that packet is completed (that is, the period of time until time T8) is written in a duration field, an address of the destination communication station (STA1) is written in a RA field, and an address thereof (STA0) is written in a TA field.
Hereupon, an attention should be paid to the point that the communication station STA0 needs to determine the time when the transaction is completed at the time when RTS is transmitted, and therefore, it is necessary to make a transmission rate mode for a CTS packet, a data packet and an ACK packet which are hereinafter transmitted and received become definite at the time when the RTS is transmitted.
This RTS packet is also received by the communication station STA2 which is located in the neighborhood of the communication station STA0. When the RTS signal is received, the communication station STA2 starts a receiving operation on discovering a preamble, and further decodes PSDU based on information obtained by decoding a PLCP header. It is recognized from the frame control field in the PSDU that the packet is the RTS packet and it is learned that the communication station STA0 is to transmit some information. Furthermore, it is recognized from the RA field that the communication station STA2 is not a destination communication station. Then, in order not to interrupt the transmission of the communication station STA0, the communication station STA2 recognizes without monitoring a medium that the medium is in an occupied state and stops transmission until the transaction is completed, that is, until the time T8 when the transmission of ACK is completed in the example shown in FIG. 15. This operation is called setting up NAV (Network Allocation Vector) or the like. In the state where NAV is set up, the NAV becomes effective over a period which is indicated in the duration field and the communication station STA2 becomes in a transmission disapproval state until the time T8.
On the other hand, this RTS packet is also received by the communication station STA1 which is the destination thereof. When by decoding PSDU the communication station STA1 recognizes similarly to the above procedure that the communication station STA0 is to transmit the packet addressed thereto, the STA1 returns the CTS packet at time T3 after an SIFS interval. The transmission rate mode of the CTS packet should be the same as that of the RTS. Moreover, it is written in the frame control field of PSDU that the packet is the CTS packet, a period of time until the transaction is completed (that is, the period of time until the time T8) is written in the duration field, and the address of the destination communication station (STA1) is written in the RA field.
This CTS packet is also received by the communication station STA3 which is located in the neighborhood of the communication station STA1. The communication station STA3 recognizes that “a certain nearby communication station expects to receive the packet until the time T8” by decoding PSDU using the procedure similar to the above. Then, in order not to interrupt the reception by the communication station STA1, the communication station STA3 sets up NAV to stop transmission until that transaction is completed. The NAV becomes effective over the period indicated in the duration field, and the communication station STA3 also becomes in a transmission disapproval state until the time T8.
On the other hand, this CTS packet is also received by the communication station STA0 which is the destination thereof. The communication station STA0 recognizes by decoding PSDU using the procedure similar to the above that the communication station STA1 is ready for the reception and starts transmission of a data packet at time T5 after the SIFS interval. When the transmission of the data packet is completed at time T6 and the communication station STA1 decodes the data without an error, the ACK is returned at time T7 after the SIFS interval and the communication station STA0 receives the ACK to complete the transmission and reception transaction for one packet at the time T8. When the time T8 has come, the communication stations STA2 and STA3 which are the neighboring communication stations take the NAV down so as to return to a normal transmission and reception state.
In brief, by exchanging the above described RTS packet and CTS packet, transmission is prohibited in “the neighboring stations to the STA0 that is the transmitting station” which received the RTS and is prohibited in “the neighboring station to the STA1 that is the receiving station” which received the CTS, so that the transmission of information addressed to the communication station STA1 from the communication station STA0 and also the return of ACK are performed without being interfered by a sudden transmission signal transmitted from neighboring stations.
Japanese Published Patent Application No. H8-98255 discloses a conventional example of such access control of wireless communications.
In the meantime, in the IEEE802.11 it has been necessary to make a transmission rate of RTS, CTS and a data packet determined at the time when the RTS is transmitted, in order to write in a duration field of the RTS a period of time until a transmission and reception transaction is completed for the packet. However, the following problems may occur according to this procedure.
Problem 1: a Reaching Range of (an RTS Packet and) a CTS Packet
Although transmission of a CTS packet should normally be addressed to all communication stations which have a possibility to interrupt reception of a data packet, according to the IEEE802.11 standard the CTS packet needs to be transmitted by the same transmission rate as the data packet and the transmission is performed between a transmitting station and a receiving station only at a transmission rate having the minimum necessary noise-proof characteristics. Therefore, since the CTS packet can be delivered only to communication stations which exist within the range up to a distance equivalent to that of the transmitting station when viewing from the receiving station (CTS transmitting station), the problem of hidden terminals cannot solved fundamentally. Further, the same can also be said with respect to the RTS packet.
Problem 2: (an Influence of a Transmission Disapproval Interval NAV)
Further, according to the IEEE 802.11, a terminal which has received an RTS/CTS packet addressed to the other terminals is made to stop transmission processing (NAV) until a transaction is completed; however, actually even communication not affecting the reception in the terminal which has transmitted the CTS has been restricted. Due to the operation, a usability of a line has not been improved.
Problem 3: an Influence when a CTS Packet has not been Returned
When “a destination station of a data packet” has not been able to receive an RTS packet correctly or when it has been set to a transmission disapproval state due to some reason, transmission of the data packet is not performed, because a CTS packet is not returned to “a transmission source station of the data packet”. However, communication stations which have received the RTS packet in the neighborhood enter the transmission disapproval state until a transmission and reception transaction of the data packet is completed regardless of whether the CTS is returned or not, which is inconvenient.
Problem 4: an Imperfect Nature of a Transmission Rate
Although it is necessary to make a transmission rate of a data packet determined before an RTS packet is transmitted, a possibility that the data packet is transmitted at an optimal transmission rate corresponding to a receiving condition of a receiving station is low due to the fact that a transmitting station is not capable of obtaining the real-time receiving condition in the receiving station.
The present invention is made in light of the above problems and aims to solve problems when performing an access control in a communication system such as a wireless LAN system.